Enhancement of Fischer-Tropsch Process for Hydrocarbon Fuel Formulation in a GTL Environment

ABSTRACT

An enhanced natural gas processing method using Fischer-Tropsch (FT) process for the synthesis of sulfur free, clean burning, hydrocarbon fuels, examples of which include syndiesel and aviation fuel. A selection of natural gas, separately or combined with portions of natural gas liquids and FT naphtha and FT vapours are destroyed in a syngas generator and used or recycled as feedstock to an Fischer-Tropsch (FT) reactor in order to enhance the production of syndiesel from the reactor. The process enhancement results are the maximum production of formulated syndiesel without the presence or formation of low value by-products.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/966,952 filed Dec. 11, 2015, now abandoned, which is a continuationof U.S. application Ser. No. 13/887,342 filed May 5, 2013, which issuedas U.S. Pat. No. 9,212,342 Dec. 15, 2015, and claims priority toCanadian Patent Application No. 2,776,369 filed May 9, 2012, whichissued as Canadian Patent No. 2,776,369 Jan. 21, 2014.

FIELD OF THE INVENTION

The present invention relates to the modification of the Fischer-Tropschsequence of operations including the Fischer-Tropsch process for theproduction of hydrocarbon fuels in an efficient manner.

BACKGROUND OF THE INVENTION

In the prior art, the Fischer-Tropsch process has been used for decadesto assist in the formulation of hydrocarbons. In the last several years,this has become a concern giving the escalating environmental concernsregarding pollution together with the increasing costs of hydrocarbonexploration and refining and the increasing surplus supply of naturalgas. The major producers in this area have expanded the artsignificantly in this technological area with a number of patentedadvances and pending applications in the form of publications.

In the art, advances made in terms of the raw materials that have beenprogenitor materials for the Fischer-Tropsch process, have included, forexample, coal-to-liquid (CTL), bio-to-liquid (BTL) and gas-to-liquid(GTL). One of the more particularly advantageous features of thegas-to-liquid (GTL) technology is the fact that it presents apossibility to formulate a higher value environmentally beneficialsynthetic diesel product or syndiesel from stranded natural gas andnatural gas liquid reserves, which would otherwise have not beencommercially or otherwise feasible to bring to market. As is generallyknown, the Fischer-Tropsch (FT) process converts hydrogen and carbonmonoxide (commonly known as syngas) into liquid hydrocarbon fuels,examples of which include synthetic diesel, naphtha, kerosene, aviationor jet fuel and paraffinic wax. As a precursory step, the natural gasand natural gas liquids are thermally converted using heat and pressurein the presence of catalyst to produce a hydrogen rich syngas containinghydrogen and carbon monoxide. As a result of the Fischer-Tropschtechnique, the synthetic fuels are very appealing from an environmentalpoint of view, since they are paraffinic in nature and substantiallydevoid of contamination. This is particularly true in the case of thediesel fuel synthesis where the synthetic product has ideal propertiesfor diesel engines, including extremely high cetane rating >70,negligible aromatics and sulphur content, in addition to enablingoptimum combustion and virtually emission free operation. Syntheticdiesel or syndiesel fuels significantly reduce nitrous oxide andparticulate matter and are an efficient transportation fuel with lowergreen house gas (GHG) emissions, when compared with petroleum baseddiesel fuel and other transportation fuels. The syndiesel fuels can alsobe very effective in that they can be added to petroleum based dieselfuels to enhance their performance.

One example of recent advances that have been made in this area oftechnology includes the features taught in U.S. Pat. No. 6,958,363,issued to Espinoza, et al., Oct. 25, 2005. In the document, Espinoza etal. provide for hydrogen use in a GTL plant.

In essence, the patent teaches a process for synthesizing hydrocarbonswhere initially, a synthesis gas stream is formulated in a syngasgenerator. The synthesis gas stream comprises primarily hydrogen andcarbon monoxide. The process involves catalytically converting thesynthesis gas stream in a synthesis reaction to produce hydrocarbons andwater followed by the generation of hydrogen-rich stream in the hydrogengenerator. The process indicates that the hydrogen generator is separatefrom the syngas generator (supra) and that the hydrogen generatorcomprises either a process for converting hydrocarbons to olefins, aprocess for catalytically dehydrogenating hydrocarbons, or a process forrefining petroleum, and a process for converting hydrocarbons to carbonfilaments. The final step in the process in its broadest sense, involvesconsumption of hydrogen from the hydrogen-rich stream produced in one ormore processes that result and increase value of the hydrocarbons or theproductivity of the conversion of the hydrocarbons from the earliersecond mentioned step.

Although a useful process, it is evident from the disclosure of Espinozaet al. that there is a clear intent to create olefins such as ethyleneand propylene for petrochemical use, and aromatics for gasolineproduction. Additionally, there is a reforming step indicated to includethe reformation of naphtha feedstock to generate a net surplus hydrogenby-product which is then recombined into the process. The naphtha issubsequently converted to aromatics for high octane gasoline blendstock. There is no specific contemplation and therefore no discussion ofeffectively destroying the naphtha for purposes of enhancing theFischer-Tropsch process which, in turn, results in the significantaugmentation of hydrocarbon synthesis.

The Espinoza et al. process is an excellent gas to a liquid process linkto gasoline production from natural gas using naphtha reformation tomake the gasoline product. In the disclosure, it was discovered that theexcess hydrogen could be used to enhance the productivity of conversion.

A further significant advancement in this area of technology is taughtby Bayle et al., in U.S. Pat. No. 7,214,720, issued May 8, 2007. Thereference is directed to the production of liquid fuels by aconcatenation of processes for treatment of a hydrocarbon feedstock.

It is indicated in the disclosure that the liquid fuels begin with theorganic material, typically biomass as a solid feedstock. The processinvolves a stage for the gasification of the solid feedstock, a stagefor purification of synthesis gas and subsequently a stage fortransformation of the synthesis gas into a liquid fuel.

The patentees indicate in column 2 the essence of the technology: [0011]“A process was found for the production of liquid fuels starting from asolid feedstock that contains the organic material in which:

-   -   a) The solid feedstock is subjected to a gasification stage so        as to convert said feedstock into synthesis gas comprising        carbon monoxide and hydrogen,    -   b) the synthesis gas that is obtained in stage a) is subjected        to a purification treatment that comprises an adjustment for        increasing the molar ratio of hydrogen to carbon monoxide,        H2/CO, up to a predetermined value, preferably between 1.8 and        2.2,    -   c) the purified synthesis gas that is obtained in stage b) is        subjected to a conversion stage that comprises the        implementation of a Fischer-Tropsch-type synthesis so as to        convert said synthesis gas into a liquid effluent and a gaseous        effluent,    -   d) the liquid effluent that is obtained in stage c) is        fractionated so as to obtain at least two fractions that are        selected from the group that consists of: a gaseous fraction, a        naphtha fraction, a kerosene fraction, and a gas oil fraction,        and    -   e) at least a portion of the naphtha fraction is recycled in        gasification stage.”

Although a meritorious procedure, the overall process does not result inincreased production of hydrocarbons. The naphtha recycle stream that isgenerated in this process is introduced into the gasification stage.This does not directly augment the syngas volume to the Fischer-Tropschreactor which results in increased volumes of hydrocarbons beingproduced giving the fact that the feedstock is required for the process.To introduce the naphtha to the gasification stage as taught in Bayle etal., is to modify the H₂/CO ratio in the gasification stage using anoxidizing agent such as water vapour and gaseous hydrocarbon feedstockssuch as natural gas with the recycled naphtha, while maximizing the massrate of carbon monoxide and maintain sufficient temperature above 1000°C. to 1500° C. in the gasification stage to maximize the conversion oftars and light hydrocarbons.

In U.S. Pat. No. 6,696,501, issued Feb. 24, 2004, to Schanke et al.,there is disclosed an optimum integration process for Fischer-Tropschsynthesis and syngas production.

Among other features, the process instructs the conversion of naturalgas or other fossil fuels to higher hydrocarbons where the natural gasor the fossil fuels is reacted with steam and oxygenic gas in areforming zone to produce synthesis gas which primarily containshydrogen, carbon monoxide and carbon dioxide. The synthesis gas is thenpassed into a Fischer-Tropsch reactor to produce a crude synthesiscontaining lower hydrocarbons, water and non-converted synthesis gas.Subsequently, the crude synthesis stream is separated in a recovery zoneinto a crude product stream containing heavier hydrocarbons, a waterstream and a tail gas stream containing the remaining constituents. Itis also taught that the tail gas stream is reformed in a separate steamreformer with steam and natural gas and then the sole reformed tail gasis introduced into the gas stream before being fed into theFischer-Tropsch reactor.

In the reference, a high carbon dioxide stream is recycled back to anATR in order to maximize the efficiency of the carbon in the process. Itis further taught that the primary purpose of reforming and recyclingthe tail gas is to steam reform the lower hydrocarbons to carbonmonoxide and hydrogen and as there is little in the way of lighthydrocarbons, adding natural gas will therefore increase the carbonefficiency. There is no disclosure regarding the destruction of naphthain an SMR or ATR to generate an excess volume of syngas with subsequentrecycle to maximize hydrocarbon production. In the Schanke et al.reference, the patentees primarily focused on the production of the highcarbon content syngas in a GTL environment using an ATR as crudesynthesis stream and reforming the synthesis tail gas in an SMR withnatural gas addition to create optimum conditions that feed to theFischer-Tropsch reactor.

In respect of other progress that has been made in this field oftechnology, the art is replete with significant advances in, not onlygasification of solid carbon feeds, but also methodology for thepreparation of syngas, management of hydrogen and carbon monoxide in aGTL plant, the Fischer-Tropsch reactors management of hydrogen, and theconversion of biomass feedstock into hydrocarbon liquid transportationfuels, inter alia. The following is a representative list of other suchreferences. This includes: U.S. Pat. Nos. 7,776,114; 6,765,025;6,512,018; 6,147,126; 6,133,328; 7,855,235; 7,846,979; 6,147,126;7,004,985; 6,048,449; 7,208,530; 6,730,285; 6,872,753, as well as UnitedStates Patent Application Publication Nos. US2010/0113624;US2004/0181313; US2010/0036181; US2010/0216898; US2008/0021122; US2008/0115415; and US 2010/0000153.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improvedFischer-Tropsch based synthesis process for synthesizing hydrocarbonswith a substantially increased yield.

In one embodiment of the present invention there is provided a processfor synthesizing hydrocarbons, comprising:

-   -   a) formulating a hydrogen rich stream with a syngas generator;    -   b) catalytically converting said stream to produce hydrocarbons,        containing at least naphtha;    -   c) recycling at least a portion of said naphtha to said syngas        generator to form an enhanced hydrogen rich stream; and    -   d) re-circulating said enhanced hydrogen rich stream from        step (c) for conversion in step (b) to enhance the synthesis of        hydrocarbons.

The present technology provides a very elegant solution to amelioratethe shortcomings that have been clearly evinced in the prior artreferences. Despite the fact that the prior art, in the form of patentpublications, issued patents, and other academic publications, allrecognize the usefulness of a Fischer-Tropsch process, steam methanereforming, autothermal reforming, naphtha recycle, and other processes,the prior art when taken individually or when mosaiced is deficient aprocess that provides for the synthesis of a hydrogen rich stream in asyngas generator and reaction in a Fischer-Tropsch or suitable reactorfor the purpose of enhancing the production of, as one example, dieselfuel or aviation fuel. As is well known, the Fischer-Tropsch process isparticularly useful since the resultant synthetic fuel is “clean” fueland does not have the contamination level typically associated with thesame petroleum based fuel.

The present invention amalgamates, in a previously unrecognizedcombination, a series of known unit operations into a much improvedsynthesis route for production of synthetic hydrocarbon fuels. Thisprocess engages a counter-intuitive step, namely, the removal of aproduction fraction, namely the naphtha, which, despite being a refinedproduct, is then effectively destroyed making use of the naphtha as afeedstock for a syngas generator and then recycled into theFischer-Tropsch process. This keystone unit operation is propitioussince it works in concert with all of the other precursor operationswhich, of their own right, are highly effective.

It has been discovered that by employing the naphtha product fraction asa recycled feedstock to the syngas generator, shown in the example anddiscussed hereinafter in greater detail, as an autothermal reformer(ATR) or steam methane reformer (SMR) or combination thereof, results inan increase in the volume of diesel, or as it is more effectivelyreferred to in the art, as syndiesel.

In accordance with an embodiment of the instant methodology, the processmay include an autothermal reforming unit (ATR) operation as a syngasgenerator. As is well known to those skilled in the art, autothermalreforming employs carbon dioxide and oxygen, or steam, in a reactionwith light hydrocarbon gases like natural gas to form syngas. This is anexothermic reaction in view of the oxidation procedure. When theautothermal reformer employs carbon dioxide, the hydrogen to carbonmonoxide ratio produced is 1:1 and when the autothermal reformer usessteam, the ratio produced is approximately 2.5:1. One of the moresignificant benefits of using the ATR is realized in the variability ofthe hydrogen to carbon monoxide ratio.

The reactions that are incorporated in the autothermal reformer are asfollows:

2CH₄+O₂+CO₂→3H₂+3CO+H₂O+HEAT

When steam is employed, the reaction equation is as follows:

4CH₄+O₂+2H₂O+HEAT→10H₂+4CO

In accordance with a further embodiment of the instant methodology, theprocess may include a steam methane reformer (SMR) operation as a syngasgenerator. As is well known to those skilled in the art, steam methanereforming employs steam in a reaction with light hydrocarbon gases likenatural gas and pre-reformed naphtha to form syngas in an indirect firedheater configuration. This is an endothermic reaction where externalheat energy is required to support the reaction.

The primary reaction that is incorporated in the steam methane reformeris as follows:

Natural Gas+Naphtha+Steam+Heat→CO+nH₂+CO₂

With the steam methane reformer, the hydrogen to carbon monoxide ratioproduced ranges from 3:1 to 5:1. One of the more significant benefits ofusing the SMR is realized in the capability of generating relativelyhigh hydrogen to carbon monoxide ratios, particularly attractive whereexcess hydrogen is needed for other operations, such as for thehydrocarbon upgrader.

A further discovery materialized from making use of, for example, lighthydrocarbon gas as by-product from the Fischer-Tropsch reaction andhydrocarbon upgrader processing, commonly known as FT Tailgas andUpgrader offgases, or combined to form a refinery fuel gas, as arecycled feedstock to the ATR, SMR or combination thereof together withthe naphtha recycle feedstock, resulted in a significant increase in thevolume of syndiesel fuel produced. By way of example, by employing thecombination of SMR and ATR with naphtha recycle, and the recycledrefinery fuel gases, the process is capable of converting at least 50%or greater of all the carbon introduced to the process to syndiesel withan increase in production of syndiesel and synthetic jet fuel, ascompared to conventional Fischer-Tropsch operation and without theproduction of any hydrocarbon by-products. This obviously hassignificant economic benefits.

Accordingly, a further aspect of one embodiment of the present inventionis to provide a process for synthesizing hydrocarbons, comprising thesteps of: providing a source of hydrocarbons at least containingnaphtha, recycling the naphtha to a syngas generator to form hydrogenrich stream; and catalytically converting the hydrogen rich stream tosynthesize hydrocarbons.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided an improved gas to liquids circuit, theimprovement comprising: recycling formed naphtha to the syngas generatorto form a hydrogen rich stream with subsequent catalytic conversion.

With the broad applicability of the technology discussed herein, theamalgamation of the GTL process to a conventional hydrocarbon liquidsextraction plant facilitates transformation of the low value natural gasbyproducts to beneficially economic synthetic fuels.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided a method for converting natural gasbyproducts to synthetic fuel, comprising: providing a source of naturalgas containing byproducts, extracting byproduct fractions from thenatural gas; and converting at least a portion of the fractions tosynthetic fuel by use as a feedstock to a fuel synthesis circuit.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided a method for converting natural gasbyproducts to at least one of synthetic diesel and synthetic jet fuel,comprising: providing a source of natural gas, providing a hydrocarbonextraction gas plant and a gas to liquids plant incorporating aFischer-Tropsch reactor; extracting, in said hydrocarbon extraction gasplant, a gas phase and a hydrocarbon liquid phase from the natural gas;fractioning the hydrocarbon liquid phase to generate methane, ethane,propane, butane and pentanes plus (commonly referred to as condensate)and mixtures thereof as a feedstock; feeding the feedstock to the gas toliquids plant for reaction in the Fischer-Tropsch reactor; andconverting at least a portion of the feedstock to at least one of thesynthetic diesel and synthetic jetfuel.

By augmenting the natural gas with a secondary or ancillary feedstockfuel such as a natural gas byproduct or combination of some or allthereof, significant yield increases in synthetic fuel production havebeen realized. In this manner, the low value byproducts used as afeedstock in an integral GTL and hydrocarbon liquid extraction plant areof particular benefit.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided a method for increasing the volume yield ofsyndiesel produced in a gas to liquids processing circuit having syngasgenerator, syngas conditioning circuit and upgrading circuit,comprising: providing a source of natural gas; generating methane,ethane, propane, butane, condensate and mixtures thereof from at leastone of a portion of the source of natural gas as an ancillary feedstockfor the syngas generator; feeding the ancillary feedstock to the syngasgenerator in addition to the natural gas; and formulating syndiesel in ayield greater than in the absence of introduction of the ancillaryfeedstock into the syngas generator.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided a method for increasing the volume yield ofsyndiesel produced in a gas to liquids processing circuit having syngasgenerator, syngas conditioning circuit and upgrading circuit,comprising: providing a source of natural gas; generating methane,ethane, propane, butane, condensate and mixtures thereof from at leastone of a portion of the source of natural gas as primary feedstock forthe syngas generator; feeding the primary feedstock to the syngasgenerator; and formulating syndiesel in a yield greater than in theabsence of introduction of the primary feedstock into the syngasgenerator.

In accordance with a further aspect of one embodiment of the presentinvention, there is provided a process for synthesizing hydrocarbons,comprising the steps of: providing a source of natural gas containingbyproducts; extracting byproduct fractions from the natural gas;providing at least a portion of any of the fractions for use as afeedstock to a syngas generator formulating a hydrogen rich stream witha syngas generator; catalytically converting the stream to producehydrocarbons, containing at least naphtha; recycling at least a portionof the naphtha to the syngas generator to form an enhanced hydrogen richstream; and re-circulating the enhanced hydrogen rich stream forconversion into enhance the synthesis of hydrocarbons.

Copious advantages flow from practicing the technology of thisapplication, exemplary of which are: a) high quality diesel product oradditive; b) high quality diesel and jet fuel with an absence of sulfur;c) absence of petroleum by-products or low value feedstocks such asnaphtha, ethane, propane and butane; d) low emission and clean burningdiesel and jet fuel; e) increased cetane rating with concomitantaugmented performance; f) significant volume output of diesel/jet fuelcompared to conventional processes using a Fischer-Tropsch reactor; g)use of natural gas byproducts for synthesizing high quality syntheticfuels; and h) increased yield of synthetic fuel production by use ofnatural gas byproducts with or without natural gas.

Referring now to the drawings as they generally describe the invention,reference will now be made to the accompanying drawings illustratingpreferred embodiments and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of methodology known in the prior artusing autothermal reformer technology;

FIG. 2 is a process flow diagram of methodology known in the prior artusing steam methane reformer technology;

FIG. 3 is a process flow diagram similar to FIG. 1, illustrating a firstembodiment of the present invention;

FIG. 4 is a process flow diagram similar to FIG. 2, illustrating afurther variation of the present invention;

FIG. 5 is a process flow diagram of a still further embodiment of thepresent invention showing the combination of autothermal and steammethane reforming technologies;

FIG. 6 is a process flow diagram illustrating a still further variationof the present methodology, showing the integration of the autothermaland steam methane technologies;

FIG. 7 is a schematic diagram illustrating a conventional hydrocarbonliquids extraction plant; and

FIG. 8 is a process flow diagram illustrating a still further variationof the present methodology within a natural gas processing facility.

Similar numerals employed in the figures denote similar elements.

The dashed lines used in the Figures denote optional operations.

INDUSTRIAL APPLICABILITY

The present invention has applicability in the fuel synthesis art.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, to illustrate prior art, shown is a processflow diagram of a circuit for converting gas-to-liquids with the resultbeing the production of naphtha and syndiesel. The process is generallydenoted by numeral 10 and begins with a natural gas supply 12, whichfeedstock can be in the form of raw field gas or pipeline qualitytreated gas, usually with bulk sulfur and hydrocarbon liquids removed.The natural gas is then pre-treated in a pre-treatment unit 20 to whichsteam 14, hydrogen 18 and optionally carbon dioxide 19 may be added asrequired. The pre-treatment unit may include, as is well known to thoseskilled in the art, such unit operations as a feed gas hydrotreater,sulfur removal and guard operation and a pre-reformer to produce a cleanvapour feed stream 22 for the syngas generator, denoted in FIG. 1 as anautothermal reformer (ATR) unit 24. The ATR 24 may be any suitablecatalytic partial oxidization unit, however, as an example, an ATR thatis useful in this process is that of Haldor Topsoe A/S, Uhde GmbH andCB&I Lummus Company. The ATR process and apparatus have been found to beeffective in the methodology of the present invention and will bediscussed hereinafter.

Generally, as is known from the ATR process, the same effectivelyinvolves a thermal catalytic stage which uses a partial oxygen supply 16to convert the preconditioned natural gas feed to a syngas 26 containingprimarily hydrogen and carbon monoxide.

The so formed syngas is then subjected to cooling and cleaningoperations 28 with subsequent production of steam 32 and removal ofproduced water at 34. Common practice in the prior art is to employ theuse of a water gas shift reaction (WGS) on the clean syngas 30 tocondition the hydrogen to carbon dioxide ratio to near 2.0:1 for optimumconditions for the Fischer-Tropsch unit 40. It is not preferred in thisprocess to include a WGS reaction as all the carbon, primarily as CO isretained and used to maximize production of synthesis liquids product.The process may optionally use the supplemental addition of hydrogen 42to maximize the conversion to syndiesel. The raw syngas may be furthertreated, as is well known to those skilled in the art, in various stepsof scrubbing units and guard units to remove ammonia and sulfurcompounds to create a relatively pure clean syngas 30 suitable for usein a Fischer-Tropsch unit. A carbon dioxide removal unit (not shown) mayoptionally be included in the clean syngas stream 30 to reduce the inertload and maximize the carbon monoxide concentration to theFischer-Tropsch unit 40. The syngas is then transferred to aFischer-Tropsch reactor 40 to produce the hydrocarbons and water. The soformed hydrocarbons are then passed on to a product upgrader, generallydenoted as 50, and commonly including a hydrocarbon cracking stage 52, aproduct fractionating stage 60 with naphtha being produced at 66 as afraction, as well as diesel 68 as an additional product. The diesel 68formulated in this process is commonly known as syndiesel. As anexample, this process results in the formulation of 1000 barrels per day(bbl/day) based on 10 to 15 thousand standard cubic feet/day (MSCFD) ofnatural gas. As is illustrated in the flow diagram, a source of hydrogen74 is to be supplemented to the hydrocarbon cracking unit 52 denoted asstreams 54. Further, energy 32 from the syngas generator 24, typicallyin the form of steam, may be used to generate power and this is equallytrue of the Fischer-Tropsch reactor 40 creating energy 46.

Table 1 establishes a comparison between FT diesel and conventionalpetroleum based diesel.

TABLE 1 Specification of FT-diesel in comparison to conventional dieselConventional Diesel Fuel Specification FT-Diesel Diesel Chemical formulaParaffin C₁₂H₂₆ Molecular weight (kg/kmol) 170-200 Cetane number >74 50Density (kg/l) at 15° C. 0.78 0.84 Lower Heating Value (MJ/kg) at 15° C.44.0 42.7 Lower Heating Value (MJ/l) at 15° C. 34.3 35.7 Stoichiometricair/fuel ratio (kg air/kg fuel) 14.53 Oxygen content (% wt) ~0   0-0.6Kinematic viscosity (mm²/s) at 20° C. 3.57 4 Flash point (° C.) 72 77Source: KMITL Sci. Tech. J. Vol. 6 No. 1 January-June 2006, p. 43

As a further benefit, known to those skilled in the art, the process asdescribed by FIG. 1 and all configurations of the current invention, theaddition of a further side stripper column (not shown) off thefractionation in stage 60 may be included to produce a new fraction ofabout 25% of the volume of the syndiesel fuel (200 to 300 barrels perday (bbl/day)), referred to as FT-jet fuel. Table 2 describes a typicalcharacteristic of FT jet fuel.

TABLE 2 Typical Specification of FT-Jet Fuel Typical ProductSpecification FT Jet Fuel Acidity mg KOH/g 0.10 Aromatics % vol max<25.0 Sulfur mass % <0.40 Distillation ° C. Min 125° C. max 50%recovered 190° C. End Point 270° C. Vapor Pressure kPa max 21 FlashPoint ° C. — Density 15° C., kg/m3 750-801 Freezing Point° C. max −51Net Heat Combustion MJ/kg min 42.8 Smoke Point mm, min 20 Naphthalenesvol % max <3.0 Copper Corrosion 2 hr @ 100° C., max rating No 1 ThermalStability Filter Pressure drop mmHg, max 25 Visual Tube rating, max <3Static Test 4 hr @ 150° C. mg/100 ml, max — Existent Gum mg/100 ml, max—

Naphtha 66 can be generally defined as a distilled and condensedfraction of the Fischer-Tropsch FT hydrocarbon liquids, categorized byway of example with a typical boiling range of −40° C. to 200° C., morepreferred 30° C. to 200° C., and more preferred 80° C. to 120° C. Thespecific naphtha specification will be optimized for each application tomaximize syndiesel production, maximize the recovery of light liquidhydrocarbon fractions such as propane and butane and partially or fullyeliminate the naphtha by-product.

Suitable examples of FT reactors include fixed bed reactors, such astubular reactors, and multiphase reactors with a stationary catalystphase and slurry-bubble reactors. In a fixed bed reactor, the FTcatalyst is held in a fixed bed contained in tubes or vessels within thereactor vessel. The syngas flowing through the reactor vessel contactsthe FT catalyst contained in the fixed bed. The reaction heat is removedby passing a cooling medium around the tubes or vessels that contain thefixed bed. For the slurry-bubble reactor, the FT catalyst particles aresuspended in a liquid, e.g., molten hydrocarbon wax, by the motion ofbubbles of syngas sparged into the bottom of the reactor. As gas bubblesrise through the reactor, the syngas is absorbed into the liquid anddiffuses to the catalyst for conversion to hydrocarbons. Gaseousproducts and unconverted syngas enter the gas bubbles and are collectedat the top of the reactor. Liquid products are recovered from thesuspending liquid using different techniques such as separators,filtration, settling, hydrocyclones, and magnetic techniques. Coolingcoils immersed in the slurry remove heat generated by the reaction.Other possibilities for the reactor will be appreciated by thoseskilled.

In the FT process, H₂ and CO combine via polymerization to formhydrocarbon compounds having varying numbers of carbon atoms. Typically70% conversion of syngas to FT liquids takes place in a single pass ofthe FT reactor unit. It is also common practice to arrange the multipleFT reactors in series and parallel to achieve conversion levels of 90+%.A supplemental supply of hydrogen 42 may be provided to each subsequentFT reactor stages to enhance the conversion performance of thesubsequent FT stages. After the FT reactor, products are sent to theseparation stage, to divert the unconverted syngas and lighthydrocarbons (referred to as FT tailgas), FT water and the FT liquids,which are directed to the hydrocarbon upgrader unit denoted as 50. TheFT tailgas becomes the feed stream for subsequent FT stages or isdirected to refinery fuel gas in the final FT stage. The upgrader unittypically contains a hydrocracking step 52 and a fractionation step 60.

Hydrocracking denoted as 52 used herein is referencing the splitting anorganic molecule and adding hydrogen to the resulting molecularfragments to form multiple smaller hydrocarbons (e.g.,C₁₀H₂₂+H₂fwdarw.C₄H₁₀ and skeletal isomers+C₆H₁₄). Since a hydrocrackingcatalyst may be active in hydroisomerization, skeletal isomerization canoccur during the hydrocracking step. Accordingly, isomers of the smallerhydrocarbons may be formed. Hydrocracking a hydrocarbon stream derivedfrom Fischer-Tropsch synthesis preferably takes place over ahydrocracking catalyst comprising a noble metal or at least one basemetal, such as platinum, cobalt-molybdenum, cobalt-tungsten,nickel-molybdenum, or nickel-tungsten, at a temperature of from about550° F. to about 750° F. (from about 288° C. to about 400° C.) and at ahydrogen partial pressure of about 500 psia to about 1,500 psia (about3,400 kPa to about 10,400 kPa).

The hydrocarbons recovered from the hydrocracker are furtherfractionated in the fractionation unit 60 and refined to containmaterials that can be used as components of mixtures known in the artsuch as naphtha, diesel, kerosene, jet fuel, lube oil, and wax. Thecombined unit consisting of the hydrocracker 52 and hydrocarbonfractionator 60 are commonly known as the hydrocarbon upgrader 50. As isknown by those skilled in the art, several hydrocarbon treatment methodscan form part of the upgrader unit depending on the desired refinedproducts, such as additional hydrotreating or hydroisomerization steps.The hydrocarbon products are essentially free of sulfur. The diesel maybe used to produce environmentally friendly, sulfur-free fuel and/orblending stock for diesel fuels by using as is or blending with highersulfur fuels created from petroleum sources.

Unconverted vapour streams, rich in hydrogen and carbon monoxide andcommonly containing inert compounds such as carbon dioxide, nitrogen andargon are vented from the process as FT tail gas 44, hydrocracker (HC)offgas 56 and fractionator (frac) offgas 62. These streams can becommonly collected as refinery fuel gas 64 and used as fuel for furnacesand boilers to offset the external need for natural gas. These streamsmay also be separated and disposed of separately based on their uniquecompositions, well known to those skilled in the art.

A supplemental supply of hydrogen 74 may be required for the HC unit 54and the natural gas hydrotreater 18. This hydrogen supply can beexternally generated or optionally provided from the syngas stream 30using a pressure swing absorption or membrane unit (not shown), althoughthis feature will increase the volume of syngas required to be generatedby the syngas generator 24.

Further, useable energy commonly generated as steam from the syngasstage, denoted by numeral 32, may be used to generate electric power.This is equally true of useable energy that can be drawn from theFischer-Tropsch unit, owing to the fact that the reaction is veryexothermic and this represents a useable source of energy. This isdenoted by numeral 46.

Referring now to FIG. 2, to further illustrate the prior art, shown isan alternate process flow diagram of a circuit for convertinggas-to-liquids with the result being the production of naphtha andsyndiesel. The components of this process are generally the same as thatdescribed in FIG. 1 with the common elements denoted with the samenumbers. For this process, the syngas generator is changed to be a steammethane reformer (SMR) 25. The SMR 25 may be any suitable catalyticconversion unit, however, as an example, an SMR that is useful in thisprocess is that of Haldor Topsoe A/S, Uhde GmbH, CB&I Lummus Company,Lurgi GmbH/Air Liquide Gruppe, Technip Inc, Foster Wheeler and others.The SMR process and apparatus have been found to be effective inexecuting the methodology of the present invention to be discussedhereinafter. Generally, as is known from the SMR process, the sameeffectively involves a thermal catalytic stage which uses steam supplyand heat energy to convert the preconditioned natural gas feed to asyngas 27 containing primarily hydrogen and carbon dioxide.

An advantage of the SMR technology is that the syngas is very rich inhydrogen with a ratio of hydrogen to carbon monoxide typically greaterthan 3.0:1. This exceeds the typical syngas ratio of 2.0:1 usuallypreferred for the Fischer-Tropsch process. As such, a hydrogenseparation unit 33 may be used to provide the hydrogen requirement 74for the GTL process. As discussed previously, well known to thoseskilled in the art, the hydrogen separator may be a pressure swingadsorption or a membrane separation unit. Further, although the SMR doesnot require an oxygen source as with the ATR technology, the SMR processrequires external heat energy, typically provided by natural gas 13 oroptionally by use of the excess refinery gas 76 derived from the FT tailgas 44 or upgrader offgases 56 & 62.

The SMR 25 may contain any suitable catalyst and be operated at anysuitable conditions to promote the conversion of the hydrocarbon tohydrogen H₂ and carbon monoxide. The addition of steam and natural gasmay be optimized to suit the desired production of hydrogen and carbonmonoxide. Generally natural gas or any other suitable fuel can be usedto provide energy to the SMR reaction furnace. The catalyst employed forthe steam reforming process may include one or more catalytically activecomponents such as palladium, platinum, rhodium, iridium, osmium,ruthenium, nickel, chromium, cobalt, cerium, lanthanum, or mixturesthereof. The catalytically active component may be supported on aceramic pellet or a refractory metal oxide. Other forms will be readilyapparent to those skilled.

Turning now to FIG. 3, shown is a preliminary embodiment of thetechnology of the instant invention. As is evinced from FIG. 3, many ofthe preliminary steps are common with that which is shown in FIG. 1. Atleast a portion of the less desirable FT product, naphtha 66 is recycledas ATR 24 feed through the pre-treatment unit 20 and is fully destroyedand converted to additional syngas. Based on the full recycle andconversion of the naphtha, the diesel production increase of greaterthan 10% can be realized, with the elimination of an undesirableby-product stream.

As a key point, one of the most effective procedures in the instanttechnology, relates to the fact that once the product fractionationstage has been completed and the naphtha 66 formulated, it has beenfound that by recycle and full conversion of the naphtha, significantresults can be achieved in the production of the synthetic diesel.

In the embodiment shown in FIG. 3, several other optional features aredesirable in addition to naphtha recycle, to enhance the production ofsyndiesel, including; (i) a hydrogen separation unit is added to removeexcess hydrogen from the enhanced syngas for supply to the FT unit 40and product upgrader 50; (ii) A portion of hydrogen rich streams notdesired to be used as fuel, separately or combined all together asrefinery fuel 64, can be recycled back 102 to the ATR 24 by way of thepre-treatment unit 20; (iii) A optional carbon dioxide removal stage 21may be installed on the FT syngas feedstream to reduce the inert vapourload on the FT unit 40, and at least a portion of the carbon dioxide 12may be reintroduced into the ATR 24 by way of the pre-treatment unit 20for purposes of reverse shifting and recycling carbon to enhance theproduction of syndiesel.

As has been discussed herein previously, it is unusual and mostcertainly counter-intuitive to effectively destroy the naphtha in orderto generate a hydrogen rich stream as the naphtha is commonly desired asprimary feedstock for gasoline production. Although this is the case, itis particularly advantageous in the process as set forth in FIG. 3.

FIG. 4 sets forth a further interesting variation on the overall processthat is set forth in FIGS. 2 and 3. As is evinced from FIG. 4, many ofthe preliminary steps are common with that which is shown in FIG. 2. Inthis variation, and similar to the variation described by FIG. 3, theprocess employs the recycle of at least a portion of the naphtha 100 toenhance the production of syndiesel using a SMR syngas generator.Similarly the optional features described for FIG. 3 can equally applyto FIG. 4.

A further variation of the overall process embraced by the technologydiscussed herein is shown in FIG. 5. In essence, the process flow asshown in FIG. 5 combines the unit operations of the SMR 25 and the ATR24 syngas generators with the primary embodiment of this invention,namely the recycle of at least a portion of the naphtha, to create themaximum conversion of carbon to syndiesel. Further, the optionalfeatures as described in FIGS. 3 and 4, combined with the naphtharecycle, may create even further benefits to further enhancement ofsyndiesel production without any nonuseful by-products. The sizing ofthe ATR and SMR syngas generators are specific to each feed gascompositions and site specific parameters to optimize the production ofsyndiesel. Further the feedstreams for the ATR and SMR may be common oruniquely prepared in the pre-treatment unit to meet specific syngascompositions desired at 26 and 27. Similarly, the hydrogen rich syngasstream or portion thereof, from the SMR can be optionally preferred asthe feed stream to the hydrogen separation unit 33. By way of example,the preferred steam to carbon ratios at streams 22 and 23 for the ATRand SMR may be different, thereby requiring separate pre-treatmentsteps.

Turning to FIG. 6, as shown is yet another variation of the overallprocess according to the present invention combining the benefits ofFIGS. 3 and 4. In this embodiment, both the SMR and ATR unit operations,combined with the naphtha recycle are amalgamated into an integratedunit operation whereby the heat energy created by the ATR 24 becomes theindirect heat energy required by the SMR reactor tubes 25. Thisembodiment allows the integrated ATR/SMR unit, the XTR to bestrategically designed to maximize the carbon conversion to syndiesel bycreating the optimum Fischer-Tropsch 40 and hydrogen separator 33 syngasfeed with optimum hydrogen to carbon monoxide ratio and the minimumquantity of natural gas, steam and oxygen, while maximizing syndieselproduction without the production of any nonuseful by-product. All otheroptional features remain the same as FIGS. 3, 4 and 5. As used herein,“integrated” in reference to the ATR/SMR means a merged unit where thetwo distinct operations are merged into one.

Turning to FIG. 7, shown is a schematic illustration of a conventionalhydrocarbon liquids extraction plant commonly known in the art. Theoverall plant is denoted with numeral 110. The hydrocarbon liquidextraction gas plant typically includes refrigerated dewpoint controlunits, lean oil absorption plants or deep cut turbo expander plants. Allof these process units employ an extraction technique to remove ethane,propane, butane and pentanes as well as higher alkanes referred to aspentanes plus C₅+(typically referred as condensates) singly or as blendsfrom the methane gas stream. These techniques are well known and willnot be elaborated upon here. Generally speaking, any of the abovementioned alkanes other than the C₅+ alkanes can remain in the sales gasto increase heat content provided that the sales gas hydrocarbondewpoint specification is not exceeded.

Turning now to FIG. 8, shown is a further variation of the methodologyof the present invention. The original feedstock, namely raw natural gas114 is introduced into the plant 112 at which point the C₅+ condensatescan be removed at 116 with the passage of the methane 118, ethane 120,and propane and butane 122 introduced into a gas to liquids GTL plant124, which includes a Fischer-Tropsch unit.

As an option, at least a portion of the methane 118, ethane 120 andbutane and propane 122 can be removed as sales gas 126 or in the case ofthe ethane 120 this may be supplied optionally to the petrochemicalmarket. Similarly, with respect to the propane (C3) and butane (C4) 122this may be entirely removed or a portion thereof from the circuit at128.

As is known, once the alkane feedstock is passed into the gas to liquidplant 124 by use of the known components of the gas to liquid plantincluding, namely the syngas generator, syngas conditioning circuit, andupgrading circuit, the result is synthetic diesel fuel 130 and/orsynthetic jet fuel 132 as illustrated in the Figure.

The GTL plant 124 is capable of receiving the combined raw gas streamwith primarily the C₅+ components removed for converting the rich rawnatural gas to synthetic diesel and synthetic jet fuel. It has beenfound that over dry methane gas feed, the GTL plant 124 will generate a20% to 30% increase in synthetic diesel product yield using the richnatural gas feed. It is also been noted that a significant increase insynthetic diesel production is realized as the composition contains highconcentrations of butane and propane. It has further been found that ifthe feed is restricted to 100% propane or butane, the synthetic dieselproduction increases two to three times respectively to approximately200% to 300% of the production based on dry methane gas.

It will be appreciated that the feedstock can take any form and caninclude any combination of the byproducts or any of the byproductssingly, namely, the C₂+, C₃+, C₃ and C₄ and/or C₅+. The arrangement isparticularly beneficial, since the operator can select an option toadjust the economical business model to optimize the economics for aparticular market situation.

Clearly there are significant advantages that evolve from unifying thegas plant with the use of the byproduct technology set forth herein.These include, for example: i) Production of natural gas to be sustainedduring surplus natural gas market conditions; ii) The use ofunfavourable natural gas components (byproducts) which can be reformedto high value synthetic diesel and synthetic jet fuel to increase marketpotential; and iii) The use of rich feed streams to the GTL plant todramatically increase synthetic diesel production.

With respect to the efficiency of the overall system, in Table 3 thereis tabulated information regarding the natural gas feed and the resultof total synthetic diesel production.

TABLE 3 Overall Process Summary of GTL Pipeline Case 1 Case 2 Case 3Case 4 Natural Mixed GTL LPG Pure Pure Gas Feed Blend Propane Butane GTLFeedstock Feed Rate 12.5 12.5 12.5 12.5 12.5 (MMSCFD) Feed Composition(mole fraction) Nitrogen 0.0197 0.0 0.0 0.0 0.0 Methane 0.9700 0.8 0.00.0 0.0 Ethane 0.0010 0.0 0.0 0.0 0.0 Propane 0.0040 0.1 0.5 1.0 0.0Butane 0.0040 0.1 0.5 0.0 1.0 Pentane Plus 0.0013 0.0 0.0 0.0 0.0 TotalDiesel 996.5 1179.0 2748 2355.0 3093 Product (bpd)

As is evident from the Table, the natural gas feed to the GTL circuithas a total diesel production barrels per day (bpd) of 996.5. Cases 1though 4 vary the feed composition to the GTL circuit with verypronounced results. In the instance of Case 4 where the feed is straightbutane, the result is 3093 bpd of syndiesel which, representsapproximately a 300% increase from the use of conventional natural gaswith all of the byproducts present. Case 3 indicates straight propane asan option with an indicated total syndiesel product of 2355 bpd. Case 2demonstrates a mix between propane and butane as the feedstock, alsoillustrating a significant increase in product yield showing 2748 bpd ofsyndiesel relative to the use of natural gas only. It will beappreciated that in the instances of Cases 1 through 4, these aredemonstrative of the increase in volume of the syndiesel produced whenused in combination with the typical natural gas composition undercolumn Pipeline Natural Gas.

Clearly, the methodology facilitates an increased yield of syntheticfuel production by use of natural gas byproducts with or without naturalgas. This advantageously provides process flexibility and definitioneconomics.

1. A method for producing synthetic hydrocarbons comprising: providing a hydrocarbon source consisting essentially of ethane, propane, butane, pentane, pentane plus or mixtures thereof to a syngas generator comprising steam methane reformer under conditions to produce a hydrogen-rich syngas stream; and catalytically converting the hydrogen rich syngas stream in a Fischer-Tropsch reactor to produce synthetic hydrocarbons.
 2. The method of claim 1, wherein the hydrocarbon source is extracted from natural gas.
 3. The method of claim 2, wherein the extracted natural gas produces a gas phase consisting essentially of methane, and a liquid phase consisting essentially of ethane, propane, butane, pentane, pentane plus or mixtures thereof.
 4. The method of claim 1, wherein the hydrocarbon source consists essentially of ethane.
 5. The method of claim 1, wherein the hydrocarbon source consists essentially of propane.
 6. The method of claim 1, wherein the hydrocarbon source consists essentially of butane.
 7. The method of claim 1, wherein the hydrocarbon source consists essentially of pentane.
 8. The method of claim 1, wherein the hydrocarbon source consists essentially of pentane plus.
 9. The method of claim 1, further comprising a scrubbing unit to remove one or more components from the syngas stream.
 10. The method of claim 9, wherein the one or more components comprise one or more of ammonia, sulfur compounds, and carbon dioxide.
 11. The method of claim 1, wherein the hydrogen to carbon monoxide ratio is greater than 3:1.
 12. The method of claim 1, wherein the hydrocarbon source is provided to a combined steam methane reformer and an autothermal reformer.
 13. The method of claim 1, wherein the synthetic hydrocarbons produced comprise diesel fuel.
 14. The method of claim 1, wherein the synthetic hydrocarbons produced comprise jet fuel.
 15. The method of claim 1, wherein the synthetic hydrocarbons produced have an absence of sulfur.
 16. The method of claim 1, wherein the synthetic hydrocarbons produced have an increased cetane rating as compared to the cetane rating of petroleum based diesel.
 17. The method of claim 1, which is performed in a gas to liquid (GTL) plant.
 18. The method of claim 1, wherein said synthetic hydrocarbons contain at least naphtha; and said method further comprises recycling at least a portion of said naphtha to said steam methane reformer to form an enhanced hydrogen rich syngas stream; and re-circulating said enhanced hydrogen rich syngas stream for conversion to synthetic hydrocarbons.
 19. The method of claim 1, wherein said synthetic hydrocarbons contain at least naphtha and unconverted FT (Fischer-Tropsch) vapours; and said method further comprises recycling at least a portion of said naphtha and unconverted FT vapours to said syngas generator to form an enhanced hydrogen rich syngas stream; and re-circulating said enhanced hydrogen rich syngas stream for conversion to synthetic hydrocarbons.
 20. The method of claim 1, wherein the hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of greater than 2:1.
 21. The method of claim 1, wherein the hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of greater than 3:1.
 22. The method of claim 1, wherein the hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of approximately 2.5:1.
 23. The method of claim 1, wherein the hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of from 3:1 to 5:1.
 24. The method of claim 1, further comprising separating a portion of hydrogen from the syngas and feeding said portion to said SMR.
 25. A method for producing synthetic hydrocarbons comprising: providing a first hydrocarbon source consisting essentially of ethane, propane, butane, pentane, pentane plus or mixtures thereof; providing a second hydrocarbon source comprising natural gas, methane, naphtha or combinations thereof; providing both the first and second hydrocarbon sources to a syngas generator comprising steam methane reformer under conditions to produce an enhanced hydrogen-rich syngas stream, wherein the first and second hydrocarbon sources comprise no more than 80% methane; and catalytically converting the enhanced hydrogen rich syngas stream in a Fischer-Tropsch reactor to produce synthetic hydrocarbons in a yield greater than that produced in the absence of the first hydrocarbon source.
 26. The method of claim 25, wherein the hydrocarbon source is extracted from natural gas.
 27. The method of claim 26, wherein the extracted natural gas produces a gas phase consisting essentially of methane, and a liquid phase consisting essentially of ethane, propane, butane, pentane, pentane plus or mixtures thereof.
 28. The method of claim 25, wherein the hydrocarbon source consists essentially of ethane.
 29. The method of claim 25, wherein the hydrocarbon source consists essentially of propane.
 30. The method of claim 25, wherein the hydrocarbon source consists essentially of butane.
 31. The method of claim 25, wherein the hydrocarbon source consists essentially of pentane.
 32. The method of claim 25, wherein the hydrocarbon source consists essentially of pentane plus.
 33. The method of claim 25, further comprising a scrubbing unit to remove one or more components from the syngas stream.
 34. The method of claim 33, wherein the one or more components comprise one or more of ammonia, sulfur compounds, and carbon dioxide.
 35. The method of claim 25, wherein the hydrogen to carbon monoxide ratio is greater than 3:1.
 36. The method of claim 25, wherein the hydrocarbon source is provided to a combined steam methane reformer and an autothermal reformer.
 37. The method of claim 25, wherein the synthetic hydrocarbons produced comprise diesel fuel.
 38. The method of claim 25, wherein the synthetic hydrocarbons produced comprise jet fuel.
 39. The method of claim 25, wherein the synthetic hydrocarbons produced have an absence of sulfur.
 40. The method of claim 25, wherein the synthetic hydrocarbons produced have an increased cetane rating as compared to the cetane rating of petroleum based diesel.
 41. The method of claim 25, which is performed in a gas to liquid (GTL) plant.
 42. The method of claim 25, wherein said synthetic hydrocarbons contain at least naphtha; and said method further comprises recycling at least a portion of said naphtha to said steam methane reformer to form an enhanced hydrogen rich syngas stream; and re-circulating said enhanced hydrogen rich syngas stream for conversion to synthetic hydrocarbons.
 43. The method of claim 25, wherein said synthetic hydrocarbons contain at least naphtha and unconverted FT (Fischer-Tropsch) vapours; and said method further comprises recycling at least a portion of said naphtha and unconverted FT vapours to said syngas generator to form an enhanced hydrogen rich syngas stream; and re-circulating said enhanced hydrogen rich stream for conversion to synthetic hydrocarbons.
 44. The method of claim 25, wherein the enhanced hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of greater than 2:1.
 45. The method of claim 25, wherein the enhanced hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of greater than 3:1.
 46. The method of claim 25, wherein the enhanced hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of approximately 2.5:1.
 47. The method of claim 25, wherein the enhanced hydrogen-rich syngas stream has a hydrogen to carbon monoxide ratio of from 3:1 to 5:1. 